Print gap setting in an impact printer

ABSTRACT

An impact printer having a print head, the print head having a movable armature, a printing wire fixed to the armature, and sensor for detecting movement of the armature. The position of the armature is monitored based on an output of the sensor. A home sensor detects whether the print head is at a home position. A paper end sensor detect whether a printing medium is between the platen and the print head. A gap motor means adjusts a gap between the printing wire and the platen. A gap controller is responsive to detection signals output by said home sensor and said paper end sensor, for controlling the print head to swing the armature so as to cause the tip of said printing wire to strike the platen, determining a time of impact of the tip with the platen based on the monitored position of the armature, performing a comparison of the time of impact with a predetermined impact time, and outputting a drive signal to the gap motor for adjusting the gap according to a result of the comparison.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority benefits under 35 U.S.C §119 ofJapanese applications Ser. No. 4-280, 128, filed Oct. 19th, 1992, andSer. No. 4-330,814, filed Dec. 11th, 1992, the entire disclosures ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an impact printer, and more particularly, toan impact printer provided with a plurality of electrostatic capacitancesensors within a wire dot print head (hereinafter simply "print head"),to monitor the operations of printing wires.

2. Description of Related Art

An impact printer has a print head mounted on a carriage. Within theprint head are provided a plurality of electrostatic capacitance sensorsfor monitoring the operations of printing wires fixed to the tips ofarmatures that oppose cores of respective electromagnets. The printingwires are operative to print dots on a printing medium by driving coilsof the respective electromagnets, the coils being wound around therespective cores. The output of an electrostatic capacitance sensor areused to control the length of time that a coil is driven, thereby toimprove the quality of printing. Such impact printers also use theelectrostatic capacitance sensors for detecting respective gaps betweenthe printing medium and the printing wires. The detection of gapsserves, among other things, to distinguish the type of printing medium.For example, detection of the gap between the printing wires and theprinting medium may be used to distinguish the printing medium asbetween single sheets of paper and four-sheet-duplication paper. Suchdetection of gaps and determinations of printing medium are used toadjust the gaps to optimum values for improved printing quality.

During printing, a gap between an ink ribbon and each of the printingwires will vary depending on the type of printing medium, for example,whether the printing medium is a single sheet of paper or isfour-sheet-duplication paper. Thus, the gap is wider when the printingmedium is a single sheet of paper than when the printing medium isfour-sheet-duplication paper. As a result, the printing wires willoperate differently in each case. That is, the time duration of a strokeof a printing wire (hereinafter "impact time"), upon which the printingwire strikes the printing medium, is longer when the printing medium isa single sheet of paper than when the printing medium isfour-sheet-duplication paper. Using the relationship of impact time topaper type, the impact printer can distinguish the type of printingmedium, as well as detect the thickness of the printing medium. Theprinter adjusts the gap to have an optimum value that optimizes thecontrol of the armatures of the printer for the particular type ofprinting medium. This optimum value is established in relation to astandard gap between each printing wire and the platen of the printer atthe time that the printer is assembled.

The information which indicates the standard gap (hereinafter called"standard information value") is measured under predetermined conditionsand stored in a non-volatile memory of the printer, so that the standardinformation value can be preserved as an initially stored value even ifa power source for the printer is cut off. The standard informationvalue of conventional impact printers cannot be adjusted or renewedduring repeated use of the printer when once set to an optimum value.

In such a known impact printer, however, it is difficult to maintain theoptimum gap after repeated use of the printer. In such a printer,alternating deactivations and activations of a coil cause the strokes ofthe printing wire toward and away from the platen. Upon each stroke theattracting end of the core contacts, and therefore slowly abrades, aside of the armature. Abrasion of the armature results in a lengtheningof the stroke of the printing wire. Also, the gap between the printingwire and the platen is lengthened. Therefore, even though the gapinitially is set to an optimum value, at a time that the impact printeris built and adjusted, repeated printing cause the gap to be increasedand therefore to become greater than the optimum value.

Moreover, the optimum gap may be changed by wear of the tip of theprinting wire and of the carriage, and by external impacts exerted uponthe printer. Also, by such wear and impacts, the impact time of eachprinting wire, as detected on the basis of the output of thecorresponding electrostatic capacitance sensor, changes, therebyreducing the accuracy with which gaps are detected and with which thetypes of printing medium are distinguished.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an impact printer whichsolves the above-described problems in a conventional impact printer. Itis a further object of the invention to provide an impact printercapable of accurate gap detection, an accurate determination of the typeof printing medium, even after the parts of the printer have become wornfrom repeated use, and improved printing quality.

It is another object of the invention to provide an impact printercapable of accurate standard information value detection, and accuraterenewal of the value, even after the parts of the printer have becomeworn from repeated use.

The foregoing objects are accomplished with an impact printer having aprint head, the print head having an armature, a printing wire fixed tothe armature, an electrostatic capacitance sensor being formed from thearmature and a part fixed in opposition to the armature, the armaturebeing movable with respect to the fixed part. The impact printeraccording to the invention includes a monitoring means for monitoringthe position of the armature, based on an output of the sensor, whilethe armature is moved relative to the fixed part. The printer furtherincludes a home sensor means for detecting whether the print head is ata home position at one end of the platen, and a paper end sensor fordetecting the existence of a printing medium between the platen and theprint head. A gap motor is provided for adjusting a gap between theprinting wire and the platen. In addition, a gap control means,responsive to detection signals output by the home sensor and the paperend sensor, performs the following functions at a time of resetting thegap:

(a) control of the print head to swing the armature so that the tip ofthe printing wire strikes the platen,

(b) determination of a time of impact of the tip with the platen, basedon the monitored position of the armature,

(c) comparison of the time of impact with a predetermined impact time,and

(d) output of a drive signal to the gap motor for adjusting the gapaccording to a result of the comparison.

During the resetting period, since the gap is adjusted to a properlength, the impact printer can accurately detect the thickness and typeof the printing medium, even if the gap has changed from wear caused byrepeated use.

According to a preferred embodiment of the invention, a counter isprovided for counting the number of stokes of the printing wires andoutputting an interrupt signal for adjusting the gap each time apredetermined number of printing strokes has been counted. The gap canbe adjusted outside of an area for printing, notwithstanding theposition of the print head, and accordingly, the ink from the ink ribbonnever contacts the platen. The gap also can be adjusted while the platenis rotated, which permits the gap control means to use information aboutthe eccentricity of the platen to improve the accuracy of adjustment ofthe gap.

In another embodiment, the impact printer further includes an alarmdisplay means, a mode selecting means, and a memory means for storingdata indicative of an impact time corresponding to a predetermined gap.The mode selecting means serves for selecting either an automatic modeor a non-automatic mode, as a mode, for adjusting the gap. The modeselecting means outputs a signal indicative of the selected mode. If theautomatic mode has been selected, the gap control means automaticallycontrols an adjustment of the gap. When the non-automatic mode isselected, the gap control means activates the alarm display means whenthe gap is detected to equal or exceed a predetermined value.

According to another aspect of the invention, an impact printer has amonitoring means for monitoring the position of the armature, based onan output of the electrostatic capacitance sensor while the armature ismoved relative to a fixed part, the monitoring means including speedwaveform determining means for determining a speed waveform of thearmature while the printing wire is moved under predeterminedconditions. A detecting means is provided for detecting an operationtime of the printing wire, during which time the printing wire is movedwith respect to the fixed part, the detecting means detecting theoperation time based on the determined speed waveform. A memory storesthe detected operation time as one or more standard information values.A renewing means is provided for renewing the standard informationvalues stored in the memory. An adjusting means adjusts a gap betweenthe printing wire and the platen, according to the standard informationvalues. In a preferred embodiment, the standard information valuesinclude a release time, an impact time, and a return time of theprinting wire, because these times may differ according to suchvariables as the characteristics of the print head, the eccentricity ofthe platen, and unevenness of adjustment of the gap. The impact printercan therefore adjust the gap using the standard information values, soas to take into account the wear of the parts of the printer due torepeated use.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will beapparent to those skilled in the art from the following detaileddescription of the preferred embodiments, when considered in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating a gap control system of an impactprinter, according to a preferred embodiment of the invention;

FIG. 2 is a cross-sectional view of a print head of the impact printer,according to a preferred embodiment of the invention;

FIG. 3 is a schematic illustration of a gap between the print head and aplaten of the impact printer of FIG. 1;

FIG. 4 is a flow chart of the operation of the gap control system ofFIG. 1;

FIG. 5 is a block diagram of a gap control system of an impact printeraccording to another preferred embodiment of the invention;

FIG. 6 is a timing diagram of a speed waveform showing standardinformation values for an impact printer according to yet anotherpreferred embodiment of the invention;

FIG. 7 is a block diagram of a gap control system of an impact printeraccording to another preferred embodiment of the invention;

FIG. 8 is a partial flow chart of the operation of the gap controlsystem of FIG. 7; and

FIG. 9 is another partial flow chart of the operation of the gap controlsystem of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, there is illustrated a gap controlsystem of an impact printer according to a preferred embodiment of theinvention. The gap control system includes a gap controller 30. The gapcontroller 30 includes a memory 32, a central processing unit(hereinafter called "CPU") 31, and an input/output port (hereinaftercalled "I/O port") 33. The gap controller 30 is connected to a paper endsensor 34, a home sensor 36, a gap motor 37, a print head 38 and a groupof electrostatic capacitance sensors 20. The electrostatic capacitancesensors 20 are installed in the print head 38.

FIG. 3 is a schematic illustration of the print head 38, which attachedto a carriage 45 in opposition to a platen 21. Printing wires 15 in theprint head 38 are capable of projecting toward the platen 21 to strikean ink ribbon 22. Before printing occurs, the printing wires 15 are heldstationary, spaced from the platen 21 by a standard gap gA.

Referring to FIG. 2, there is illustrated a print head 38 of knownconstruction. In the print head 38, a permanent magnet 2, a base plate3, and a spacer 4 are stacked sequentially on the periphery of a base 1.A plate spring 7 and a yoke 8 are clamped by a clamper 6 and heldbetween the spacer 4 and a guide frame 5 so that the circumference ofthe plate spring 7 is fixed to the body of the print head 38. The platespring 7 has a plurality of bendable portions extending radiallyinwardly from its circumference. The print head also includes aplurality of electromagnets 11. Each electromagnet 11 is composed of acore 9 that stands upright on the base 1, and a coil 10 wound on thecore. A casing 12 is provided on a bottom side of the base and protectsa power supply board 13 that supplies power to each electromagnet 11. Arespective armature 14 is mounted on each of the bendable portions ofthe spring plate 7. The armatures 14 are placed adjacent to the yoke 8and in opposition to the cores 9 of the respective electromagnets 11.Fixed to the tips of the respective armatures 14 are the printing wires15. The printing wires 15 are guided through a wire guide 16 formed onthe guide frame 5.

A circuit board 17 for detection is provided between the guide frame 5and the yoke 8 and has mounted thereon a plurality of fixed electrodeends 18. The fixed electrode ends 18 oppose the armatures 14 with narrowspaces therebetween, so as to form, with detecting devices 19 on theboard 17, the respective electrostatic capacitance sensors 20, fordetecting electrostatic capacitances between the respective electrodeends 18 and armatures 14. The value of the electrostatic capacitance isdetermined in accordance with the space between the fixed electrode end18 and the armature 14, and therefore, varies according to the shifts inposition of the armature 14 during printing operations. Accordingly,each sensor 20 senses changes of the electrostatic capacitance value,thereby to detect movements of the printing wire 15 through thedetecting device 19.

Between imprints, the plate spring 7 and armature 14 are held againstthe core 9 by the magnetic force of permanent magnet 2 which bends theplate spring 7 downwardly. During printing, the electromagnet 11 isexcited to cancel the magnetic flux generated by the permanent magnet 2,thereby releasing the plate spring 7 and armature 14 from the core 9.Then, the printing wire 15 fixed to the armature 14 moves toward theplaten 21 and strikes the ink ribbon 22 to cause ink to be imprinted onthe printing medium.

Referring again to FIG. 1 by an end of the printing medium, the paperend sensor 34 detects the presence of the printing medium and outputscorresponding detection signals. The paper end sensor 34 outputs a paperend signal, for example, if there is no paper between the platen 21 andthe print head 38. The home sensor 36 is arranged at a home position ofthe print head 38 and carriage 45, to detect whether the print head 38is at the home position, located at one end of the platen. The gap motor37 serves for adjusting the gap between the platen 21 and the print head38 by moving back and forth the entire print head 38 as illustrated asarrows in FIG. 3.

Stored in the memory 32 are a control program, a standard impact time T0(set at a time when the impact printer is assembled and adjusted), and agap position for determining the rotation of the gap motor 17corresponding to the gap at that time. A reset switch 39 is connectedbetween a reset terminal 31a of the CPU 31 and ground.

FIG. 4 is a flow chart illustrating the operations performed by the gapcontrol system. Depressing the reset switch 39 grounds the resetterminal 31a of the CPU 31, thereby resetting a program counter notshown. The CPU 31 then accesses an address No. 0 of the memory 32, tostart the control program.

At step S1, a judgment is made as to whether or not a printing medium isinserted in the printer between the platen and print head, based on asignal output by the paper end sensor 34. If the printing medium isdetected, the process of the gap control program skips to the end. Ifthe printing medium is not inserted, the process progresses to step S2.

At step S2, a judgment is made as to whether or not the carriage 45 thatmounts the print head 38 is at the home position, based on a signaloutput by the home sensor 36. If the carriage is at the home position,the process of the gap control program skips to the end. If the carriageis not at the home position, the process progresses to step S3.

At step S3, the CPU 31 directs a spacing motor 42 (shown in FIG. 1) tomove the carriage 45 toward the home position. Then, at step S4, the CPU31 directs excitation of the electromagnet 11 in the print head 38 tocounteract or cancel the magnetic flux of the permanent magnet 2,thereby to release the armature 14 from the core 9. At step S5, changesof the speed of the armature 14 are detected by the electrostaticcapacitance sensor 20, and the CPU 31 determines the time duration orimpact time T from the moment that the armature 14 is released up to atime that the print wire 15 strikes the platen 21. At step S6, the CPU31 reads out an impact time T0 measured and stored as a standardinformation value, at a time that the impact printer was assembled andadjusted. The CPU 31 compares this impact time T0 with the just detectedimpact time T. If the two times are equal to each other, the processproceeds to step S9. If they are not equal to each other, the processproceeds to step S7.

At step S7, the CPU 31 directs the gap motor 37 to adjust the gap gbetween the print head 38 and the platen 21, by an incremental amount.At step S8, a judgment is made as to whether or not the carriage is atthe home position, based on a signal detected by the home sensor 36. Ifthe carriage 45 is at the home position, the process of gap controlskips to the end. If the carriage is not at the home position, theprocess returns to step S3.

At step S9, the CPU 31 controls the memory 32 to store data indicativeof the position of the gap motor 37 which corresponds to the actual gapg just measured at that time. Then, at step S10, the CPU 31 directs thespacing motor 42 to move the carriage to the home position and waits forthe home sensor 36 to detect that the carriage has reached the homeposition. Upon such detection, the gap control process is complete.

As described above, the impact time T may be measured multiple timesdepending on the initial position of the carriage 45.

Referring to FIG. 5, another preferred embodiment is shown. The gapcontroller 40 includes a CPU 31, a memory 32, and an I/O port 33, as inthe embodiment described above. The gap controller 40 is connected to apaper end sensor 34, a plurality of electrostatic capacitance sensors20, a home sensor 36, a gap motor 37, a print head 38, and a counter 41.That is, the major difference from the embodiment described above isthat the counter 41 is provided for determining when to adjust the gapbetween the platen 21 and the printing head 38. The counter 41 countsthe number of dot printing occurrences and outputs an interrupt signalto the gap controller 40 every predetermined number of such occurrences.The CPU 31 conducts the adjustment of the gap as described above at atime that the CPU 31 receives the interrupt signal.

It is to be noted that, in the embodiments described above, theadjustments of the gap can be performed without of the printing. Inother words, the adjustments can be performed while no paper existbetween the platen 21 and the print head 38. The electromagnet 11 isexcited at that time to conduct the gap adjustment described above.According to this control, the gap is adjusted again notwithstanding theposition of the printing head 38. Moreover, detection of the impact timeT during rotation of the platen 21 improves the accuracy of the gapadjustment, since information regarding the eccentricity of the platen21 is obtained from the detection.

In a modification of the embodiments described above, the memory 32stores data of impact time T corresponding to a predetermined gap g, anda gap adjusting mode selector and an alarm device are provided forconducting operations in an "Automatic Mode" and a "Non-Automatic Mode".If "Automatic Mode" is selected by the gap adjusting mode selector, thegap is automatically adjusted either at a time of resetting the CPU 31when the power supply source is turned on or every predetermined numberof printing occurrences (impacts of the printing wires). If the"Non-Automatic Mode" is selected, the gap is not adjusted automatically,and when the gap g reaches a predetermined value, the alarm device issuean alarm to the operator by a display on a display device (not shown).The user can adjust the gap through manual controls, the menu of theprinter, or through instructions from a host computer.

As described above, the impact printer thus described detects the impacttime T, i.e. the duration of time up to a time that the printing wire 15strikes the platen 21, and compares it with the impact time T0determined at the time that the impact printer is assembled andadjusted. According to an embodiment of the invention, the impactprinter also can calculate a variety of time information by slicing theoutput of the electrostatic capacitance sensors 20. Such calculatedinformation may be compared to information previously calculated andstored and the result of the comparison can be used to adjust the gap.

In the forgoing embodiments, the gap adjustment has been performed bycomparing the just measured gap information with the standardinformation value stored in the memory. The standard information valueitself has not been changed or renewed prior to the adjustment beingperformed.

FIG. 6 shows an output of the electrostatic capacitance sensor of theinvention as a function of time and time information in the form of aspeed waveform. Time information of three types is determined, under thecontrol of the CPU 31 by slicing the output of the electrostaticcapacitance sensor at a high slice level SL1 and at a low slice levelSL2.: Namely, a release time T_(RL) which is a duration time from thebeginning of the current waveform until the core 9 releases the armature14, an impact time T_(IM) from the end of the release time up to a timethat the printing wire 15 strikes the printing medium, and a return timeT_(RT) from the end of the impact time up to a time that the core 9captures the armature 14 again after impact, are obtained. The highslice level SL1 and the low slice level SL2 are used for reducing thenoise at a time that the time information is detected. Release timeT_(RL), impact time T_(IM), and return time T_(RT) of respectiveprinting wires are detected and stored in the memory 32 as the standardinformation value, at the time when the printer is assembled. Then, theimpact printer can change or renew those values based on the datasubsequently obtained.

Now, referring to FIG. 7, another preferred embodiment of an impactprinter is shown. In FIG. 7, the same reference numerals designate thesame elements as those of the embodiment described above with referenceto FIG. 1. In this embodiment, the standard information value itselfstored in advance in the RAM 32a, with which the actually obtained datais compared, is changed or renewed to reflect changes of the parts ofthe printer. The print head 38 includes a head driver 38a operating inresponse to a signal from a CPU 31 in a gap controller 50, a pluralityof coils 10 of the electromagnets for operation of the armatures 14, aplurality of electrostatic capacitance sensors 20 for detecting themovement of the respective armatures 14, and a plurality of detectingdevices 19 for outputting a detection signal to the gap controller 50.The gap controller 50 is connected to a paper end sensor 34 fordetecting existence of a printing medium between the platen and theprint head 38, and a gap motor 37 is provided for moving the printerhead 38 toward or away from the platen 21. The gap controller 50 is alsoconnected to a host computer, not shown, to receive data for printing,through an interface circuit 51. The CPU 31 in the gap controller 50 isused for performing processing of various types, such as, calculatingthe gap g based on the data of release time T_(RL), impact time T_(IM),and return time T_(RT). The CPU 31 is connected to the host computer andother devices, through interfaces 33a, 33b. Connected to the CPU 32 inthe gap controller 50 also are a RAM 32a as a backup memory for storingprinting data, the standard information values of release time T_(RL),impact time T_(IM), and return time T_(RT) for each printing wire 15,and the like, and a ROM 32b for storing the control program and fontsfor printing. The CPU 31, RAM 32a and ROM 32b also serve other functionsrelated to printing operations.

At the time that the impact printer is constructed, the data of releasetime T_(RL), impact time T_(IM), and return time T_(RT) for eachprinting wire 15 are detected under predetermined conditions, such aswith a gap g in a preset range and a known type of printing paper.Differences in the detected values of release time T_(RL), impact timeT_(IM), and return time T_(RT) will occur according to the nature of theprint head 38, the eccentricity of the platen 21, nonuniformity ofadjustment of the gap, and the like. Those values initially are storedin the RAM 32a as "standard information values". In other words, the RAM32a initially stores standard information values which take into accountthe nonuniformity of characteristics of the printer at the time theprinter is assembled.

The stored standard information values can be replaced by newinformation values, that is "renewed", in accordance with changes of thegap g. Each time the standard information values are to be renewed, thetemperature and drive voltage of the print head 38 are detected, and thenew standard information values are corrected to correspond to values ata normal temperature and a reference voltage. Such a correction isperformed, for example, at a time that the ink ribbon 22 is set or thatthe power is turned on, prior to loading the printing medium. Inaddition, the standard information values can also be replaced at a timethat the printer receives a particular function command from a hostcomputer not shown. The standard information values and new informationvalues may be outputted from the impact printer by depressing a specialkey or keys of a front panel (not shown) of the impact printer.Therefore, it is possible to store the standard information values at atime of shipping from a factory, and it is also possible to check thecondition of the impact printer by measuring new information values attimes that routine maintenance and customer service are performed, andto update the standard information values upon replacement of the printhead 38.

By setting a permissible range for the new information values, itbecomes possible, for example, to detect defects in the print head suchas folded wires, folded springs, and the like, and to detectextraordinary gaps g, when the detected values g are outside of therange. A display of an alarm at a control panel or the like (not shown),can indicate that it is time to replace the print head 38 and check thegap g since a check of the gap g and renewal of standard informationvalues is performed in a time when the printing paper is fed to theprinter. It is less frequently required in a case when the impactprinter feeds the printing paper from a rear or bottom side of theprinter, using roll paper, since the roll provides an extended source ofpaper of the same type. By comparison, the renewal of the standardinformation values is more frequently required if the impact printerfeeds single sheets of printing paper from a top side. It is thereforedesirable to control the frequency of the renewal, according to theroutes of the printing medium, by disposing counters at appropriatelocations inside of the impact printer.

Referring again to the illustration of the print head 38 in FIG. 2, wearof the core 9 due to its contact with armature 14 ordinarily issuppressed by coating the core 9 with oil. If the oil deteriorates sothat its viscosity becomes high, the armature 14 may stick to the core 9during a long time interval between use. As a result, inaccurateinformation values may be detected at a time of reuse, and also suchincorrect values may be stored at a time of renewing the standardinformation values. Furthermore, malfunctions may occur during testconducted before actual printing. To avoid such problems, the impactprinter can be caused to perform dummy impacts by initiating a smallcurrent flow initiated through the respective coils 10 for the printingwires 15, immediately before the renewal of the standard informationvalues or the test printing.

Operation of the impact printer of FIG. 7 will now be described withreference also to the flow charts of FIGS. 8 and 9.

Referring to FIG. 8, after startup, the power of the impact printer isturned on at step S11 and initialization of the CPU 31 is performed atstep S12 in a conventional manner. A judgment is made at step 13 as towhether or not a printing medium is inserted in the printer between theplaten 21 and print head 38, based on a signal output by the paper endsensor 34. If the printing medium is detected, the process for renewingthe standard information values skips to the end and the printer turnsto an actual operating mode awaiting printing data, at step S17. If theprinting medium has not been inserted, the process progresses to stepS14.

At step S14, the CPU 31 directs excitation of the electromagnet 11 inthe print head 38 to measure the standard information values, under thesame conditions as those set for printing. At step S15, the gap motor 37drives the print head 38 to a position spaced from the platen 21 by apredetermined gap g. Also at step S15, each printing wire 15 in sequenceis caused to directly strike the platen 21, and the actual release timeT_(RL), impact time T_(IM), and return time T_(RT) are detected for eachprinting wire 15. At step S16, the detected values are stored in the RAM32a as the standard information values. At the same time, the old timedata are deleted from the RAM 32a. At step S16, the standard informationvalues previously stored in the RAM 32a are replaced by new informationvalues, based on the gap g.

The above mentioned operations are performed with the ink ribbon beinginstalled. If necessary, a sensor, for detecting whether the ribbon isproperly installed or not, can be provided. Between step S15 and stepS16, some supplemental proceedings, such as temperature compensation andan abnormal value check, can be performed. At step S16, one or more, butnot all than one of the detected values can be stored in the RAM 32a.Or, other information values calculated in accordance with the detectedvalues can be also used instead of the detected values themselves.

According to the embodiment described above, the standard informationvalues already set and stored in the memory and used to determine theoptimum gap can be renewed automatically at the paper end positionwithout any additional or special manipulation by an operator when thepower is turned on so that an accurate gap adjustment can be performedbased on accurate knowledge of the paper thickness and drivingconditions of the print head 38.

Another operation of the impact printer of FIG. 7 will be described withreference to the flow chart of FIG. 9. In this operation, steps S18 andS19 are added in the process of the standard information value renewalprogram illustrated as steps S11 to S17 in FIG. 8.

Other steps S20 to S23 are also added to further proceed the gapadjustment based upon the renewed standard information values stored inthe RAM 32a at step S16. The standard information value renewal programis not so frequently performed when a roll of paper is used because thepaper is normally already set when the power happens to be turned on,whereas it is so frequently performed when a sheet of paper is used. Ifthe renewal program is performed so frequently whenever the power turnson in case of a sheet of paper, much ink is deposited on the platen 21.

To avoid the above phenomenon, a judgment is made at step S18 as towhether or not some occurrences have exceeded a predetermined number.How many times has a power supply switch turned on; how long has theprinter been used; and how many letters have been printed are consideredto be a kind of occurrences.

Accordingly at step S18, such occurrences are checked as to whether suchoccurrences fall within the predetermined number. If the occurrencefalls within the predetermined number, the process of the renewalprogram progresses to step S13. If the occurrences exceeds thepredetermined number, the process skips to the end.

At step S19, the CPU 31 reads out from the RAM 32a data representing thestandard information values measured and stored at a time the printerwas assembled and later repeatedly renewed. The CPU 31 compares thisstandard information values with the just-detected data of release timeT_(RL), impact time T_(IM), and return time T_(RT).

If the difference of two data falls within a predetermined value, thenthe process progresses to step S16, otherwise to step S20. Judgment ismade at step S20 how to proceed. If an "Alarm Procession Mode" isselected, the process progresses to step S21 where the alarm devicealarms to the operator by a display on a display device (not shown).

The operator can adjust the gap through manual controls, the menu of theprinter, or through instructions by a host computer. Then, the processprogresses to the end step S22 If a "Gap Readjusting Mode" is selected,the process progresses to step S23 where the CPU 31 directs the gapmotor 37 to adjust the gap g between the print head 38 and the platen21, by a decremental amount. Then, the process proceeds to step S15. Theselection at step S20 can be made either automatically or manually.

It is to be noted that, although in the embodiments described above theimpact printer uses a spring-charge type print head, other types ofprint heads, for example, the so called clapper-type print head, can beused for the impact printer.

It is understood that although the present invention has been describedin detail with respect to preferred embodiments thereof, persons skilledin the art will recognize various other embodiments and variations thatfall within the scope and spirit of the invention, which is limited onlyby the appended claims.

What is claimed is:
 1. An impact printer having a print head, said printhead having an armature, a printing wire fixed to said armature formovement therewith toward a platen, said platen disposed in oppositionto said print head, and a sensor for detecting movement of saidarmature, said print head being movable lengthwise of said platen withrespect to a home position at one end of said platen, said impactprinter comprising:(a) monitoring means for monitoring a position ofsaid armature, based on an output of said sensor while said armature ismoving; (b) home sensor means for detecting whether or not said printhead is at said home position; (c) a paper detector for detectingexistence of a printing medium in said printer between said platen andsaid print head; (d) gap motor means, responsive to a drive signal, foradjusting a gap between said printing wire and said platen; and (e) gapcontrol means, responsive to detection signals output by said homesensor and said paper detector, forcontrolling said print head to swingsaid armature so as to cause a tip of said printing wire to strike saidplaten while said print head is outside of an area for printing,determining a time of impact of said tip with said platen based on aposition of said armature as monitored by said monitoring means,performing a comparison of the time of impact with a predeterminedimpact time, and generating the drive signal according to a result ofthe comparison and applying the drive signal to said gap motor means toadjust the gap according to the result.
 2. An impact printer accordingto claim 1, wherein said printing head includes a plurality of printingwires, a plurality of armatures corresponding to said printing wires,and a plurality of electrostatic sensors corresponding to saidarmatures.
 3. An impact printer according to claim 1, further comprisinga counter for counting a number of times that said armature is swung,and means, responsive to a predetermined count by said counting means,for outputting an interrupt signal to actuate said gap control means toadjust the gap.
 4. An impact printer as set forth in claim 1, whereinthe printing medium is a roll of paper.
 5. An impact printer as setforth in claim 1, wherein the gap is adjusted while said platen isrotated.
 6. An impact printer as set forth in claim 1, furthercomprising:alarm display means for displaying an alarm; mode selectingmeans for selecting either an automatic mode or a non-automatic mode asa mode for adjusting the gap, and outputting a signal indicative of theselected mode; and memory means for storing data of an impact timecorresponding to a predetermined gap, wherein said gap control meanscontrols adjustment of the gap when a signal from said mode selectingmeans indicates selection of the automatic mode, and, when the signalfrom said mode selecting means indicates selection of the non-automaticmode, said gap control means activating said alarm display means if thegap is detected to be equal to the predetermined gap based on datastored in said memory means.
 7. An impact printer as set forth in claim1, wherein said sensor is an electrostatic capacitance sensor formedfrom said armature and a part fixed in opposition to said armature suchthat when said armature moves with respect to said platen said armaturemoves with respect to said part.
 8. An impact printer having a printhead movable with respect to a home position, said print head having anarmature, a printing wire fixed to said armature for reciprocal movementtherewith toward and away from a platen, said platen disposed inopposition to said print head, and a sensor for detecting movement ofsaid armature, said impact printer comprising:(a) monitoring means formonitoring a position of said printing wire, based on an output of saidsensor while said printing wire is moved relative to said platen, saidmonitoring means including speed waveform determining means fordetermining a speed waveform indicative of a speed of said printing wirewhile said printing wire is moved relative to said platen underpredetermined conditions; (b) detecting means for detecting an operationtime of said printing wire during which said printing wire is moved withrespect to said platen, based on the determined speed waveform; (c) amemory storing in advance a standard information value, the standardinformation value representing a standard operation time of saidprinting wire; (d) means forrenewing the standard information valuebased on the operation time detected by said detecting means, andreplacing the standard information value stored in advance in saidmemory by the renewed standard information value; and (e) means foradjusting a gap between said printing wire and said platen based on thestored renewed standard information value and an operation time detectedby said detecting means.
 9. An impact printer as set forth in claim 5,wherein the standard information value comprises a plurality of standardinformation values, including a release time, an impact time, and areturn time of said printing wire.
 10. An impact printer as set forth inclaim 8, wherein said sensor is an electrostatic capacitance sensorformed from said armature and a part fixed in opposition to saidarmature, said armature being movable with respect to said part, saidmonitoring means monitoring a position of said printing wire based on anoutput of said electrostatic capacitance sensor while said printing wireand said armature therewith are moved relative to said fixed part, saiddetecting means detecting the operation time of said printing wire whilesaid printing wire is moved with respect to said fixed part.
 11. Animpact printer according to claim 8, wherein said print head is movablelengthwise of said platen with respect to a home position at one end ofsaid platen, further comprising:home sensor means for detecting whethersaid print head is at said home position; and a paper detector fordetecting existence of a printing medium in said printer between saidplaten and said print head; said means for adjusting the gapincludinggap motor means, response to a drive signal, for moving theprint head to adjust the gap and gap control means, responsive todetection signals output by said home sensor and said paper detector,forcontrolling said print head to swing said armature so as to cause atip of said printing wire to strike said platen, determining a time ofimpact of said tip with said platen based on a position of said armatureas monitored by said monitoring means, performing a comparison of thetime of impact with a predetermined impact time, and generating thedrive signal according to a result of the comparison and applying thedrive signal to said gap motor means to adjust the gap according to theresult.
 12. An impact printer according to claim 11, further comprisinga counting means for counting a number of times that said armature isswung, and means, responsive to a predetermined count by said countingmeans, for outputting an interrupt signal to actuate said gap controlmeans to adjust the gap.
 13. An impact printer according to claim 11,wherein said gap control means is responsive to the detection signalsoutput by said home sensor means and said paper detector, to controlsaid print head to swing said armature so as to cause said tip of saidprinting wire to strike said platen while said print head is outside ofan area for printing.
 14. An impact printer as set forth in claim 11,further comprising:alarm display means for displaying an alarm; modeselecting means for selecting either an automatic mode or anon-automatic mode as a mode for adjusting the gap, and outputting asignal indicative of the selected mode; and memory means for storingdata of an impact time corresponding to a predetermined gap, whereinsaid gap control means controls adjustment of the gap when a signal fromsaid mode selecting means indicates selection of the automatic mode,and, when the signal from said mode selecting means indicates selectionof the non-automatic mode, said gap control means activating said alarmdisplay means if the gap is detected to be equal to the predeterminedgap based on the data stored in said memory means.
 15. A method fordriving an impact printer having a print head, the print head having anarmature, a printing wire fixed to said armature for movement therewithtoward a platen, the platen disposed in opposition to the print head,and a sensor for detecting movement of the armature, the print headbeing movable lengthwise of the platen with respect to a home positionat one end of the platen, said method comprising steps of:(a) detectingwith a home sensor whether the print head is at the home position; (b)detecting with a paper sensor the existence of a printing medium in theprinter between the platen and the print head; and (c) adjusting a gapbetween the printing wire and the platen, in response to detectionsignals respectively output by said home sensor and said paper endsensor when the detection signal output by the home sensor meansindicates that the print head is not at the home position and thedetection signal output by the paper detector indicates an absence ofpaper between the print head and the platen, includingcontrolling theprint head to swing the armature so as to cause a tip of the printingwire to strike the platen; determining a time of impact of the tip withthe platen, based on a position of the armature, performing a comparisonof the time of impact with a predetermined impact time, and outputting adrive signal for adjusting the gap according to a result of thecomparison.
 16. A method according to claim 15, further comprising thesteps ofcounting a number of times that the print wire and armature aremoved toward the platen, and initiating step c when a count reachedduring said step of counting reaches a predetermined number.
 17. Amethod for driving an impact printer having a print head movable withrespect to a home position, the print head having an armature, aprinting wire fixed to the armature for reciprocal movement therewithtoward and away from a platen, the platen disposed in opposition to theprint head, and a sensor for detecting movement of the armature, themethod comprising steps of:(a) monitoring a position of the printingwire based on an output of the sensor while the printing wire is movedrelative to the platen; (b) determining a speed waveform indicative of aspeed of the printing wire while the printing wire is moved underpredetermined conditions; (c) detecting an operation time of theprinting wire during which the printing wire is moved with respect theplaten based on the determined speed waveform; (d) storing a standardinformation value prior to said step a, the standard information valuerepresenting a standard operation time of the printing wire; (e)adjusting a gap between the printing wire and the platen disposed inopposition to the print head, based on the operation time detected bysaid detecting means and the stored standard information value stored insaid step d; (f) renewing the standard information value, based on theoperation time detected in said step c; (g) storing the renewed standardinformation value obtained in said step f in place of the standardinformation value stored in said step d; and (h) adjusting the gapbetween the printing wire and the platen disposed in opposition to theprint head, according to the renewed standard information value storedin said step g.
 18. A method according to claim 17, wherein the printhead is movable lengthwise of the platen with respect to a home positionat one end of the platen, and said step e includes the steps of:(i)detecting whether the print head is at the home position; (ii) detectingan absence of a printing medium in the printer between the platen andthe print head; and (iii) adjusting the gap when the print head isdetected away from the home position in said step i and an absence ofthe printing medium is detected in said step ii.
 19. A method accordingto claim 18, wherein said step iii of adjusting the gap includes thesteps ofcontrolling the print head to swing the armature so as to causea tip of the printing wire to strike the platen; determining a time ofimpact of the tip with the platen based on a position of the armature,performing a comparison of the time of impact with a predeterminedimpact time, and generating a drive signal according to a result of thecomparison and applying the drive signal to a gap motor, the gap motormoving the print head to adjust the gap.
 20. An impact printer having aprint head, said print head having an armature, a printing wire fixed tosaid armature for movement therewith toward a platen, said platendisposed in opposition to said print head, and a sensor for detectingmovement of said armature, said print head being movable lengthwise ofsaid platen with respect to a home position at one end of said platen,said impact printer comprising:(a) monitoring means for monitoring aposition of said armature, based on an output of said sensor while saidarmature is moving; (b) home sensor means for detecting whether or notsaid print head is at said home position; (c) a paper detector fordetecting existence of a printing medium in said printer between saidplaten and said print head; (d) gap motor means, responsive to a drivesignal, for adjusting a gap between said printing wire and said platen;and (e) gap control means, responsive to a detection signal output bysaid home sensor and a detection signal output by said paper detector,forcontrolling said print head to swing said armature so as to cause atip of said printing wire to strike said platen, determining a time ofimpact of said tip with said platen based on a position of said armatureas monitored by said monitoring means, performing a comparison of thetime of impact with a predetermined impact time, and generating thedrive signal according to a result of the comparison and applying thedrive signal to said gap motor means to adjust the gap according to theresult; wherein the detection signal output by said home sensor meansindicates that said print head is not at the home position and thedetection signal output by said paper detector indicates an absence ofpaper between said print head and the platen.